Hydraulic cements are cements that set and develop compressive strength due to a hydration reaction, and thus can be set under water. Hydraulic cements are often used for cementing pipes or casings within a wellbore. Successful cementing of well pipe and casing during oil and gas well completion requires cementitious slurries to exhibit a pumpable viscosity, good fluid loss control, minimal settling of particles and the ability to set within a practical time at elevated temperatures.
In a typical completion operation, the cementitious slurry is pumped into the well, down the inside of the pipe or casing and back up the outside of the pipe or casing through the annular space. This process seals the subterranean zones (often referred to as “zonal isolation”) in the formation and supports the casing. Under normal conditions, hydraulic cements, such as Portland cement, quickly develop compressive strength upon introduction to the well, typically within 48 hours from introduction. As time progresses, the cement develops greater strength while hydration continues.
It is common to use a retarder with the hydraulic cement in order to increase the pumping time of the cementitious slurry. In so doing, the retarder provides adequate thickening time to the cementitious slurry and thus enables placement of the slurry at its desired location. In order to minimize lost rig time, the thickening time of a cementitious slurry to attain a Bearden consistency (Bc) of 70 is most desirably from about 4 to about 5 hours.
In general, set retarders may be characterized as being low, medium or high temperature retarders depending on the bottom hole temperature encountered. In addition to increasing the pumping time of the cementitious slurry at elevated temperatures, the retarder also extends the setting time of the cementitious slurry.
Water-soluble sugars, sugar acids and their salts, borax and boric acid are known cement retarders. For instance, U.S. Pat. No. 3,100,526 discloses the use of glucoheptonic acid and salts thereof as a retarder; U.S. Pat. No. 3,053,673 discloses retarder systems containing a lignin derivative, such as a lignosulfonic acid salt, and either gluconic acid, gluconic acid delta lactone or an alkali metal, ammonium or alkaline earth metal gluconate; U.S. Pat. No. 4,065,318 discloses blends of borax, boric acid and gum arabic as retarders; U.S. Pat. No. 4,210,455 discloses set retarders of alkaline earth metal salts of sugar acids as well as alkaline earth metal salts of borate esters of sugars; and U.S. Pat. No. 4,706,755 discusses the use of borax as cement retarders.
Sugars have proven to be highly desirable as set retarders since they are environmentally safe. However, the use of sugars is restricted to low bottom hole temperatures since they break down at temperatures in excess of 250° F.
Boric acid and borax (also known as sodium tetraborate decahydrate, sodium tetraborate, sodium borate and disodium tetraborate) are considered high temperature retarders but are known to over-retard the cementitious slurry at lower temperatures. A slurry which is over-retarded contains too much retarder and thus takes a very long time to set. In some cases, an over-retarded cement slurry will not set at all. A slurry which is over-retarded increases the costs of cementing, including loss of rig time. For this reason, boric acid and borax are typically applied at high temperatures, generally in excess of 350° F.
In addition to over-retarding the slurry, boric acid and borax are not highly soluble in water at ambient temperatures. Thus, when a cementitious slurry is prepared on the fly, there typically is an abundance of non-dissolved, dispersed particulates of boric acid and borax in the slurry. When introduced downhole, shorter or uncontrolled set times and lost rig time are often seen since setting requires dissolution of the borax or boric acid in the slurry.
Further, boric acid set retarders usually contain boric acid or its equivalent in excess of 5.5%. Such amounts are in excess of established international thresholds of non-toxicity.
A need exists for a set retarder which may be applied over a broad temperature range and which does not over-retard the cementitious slurry introduced into the well.
A need further exists for a high temperature cement retarder which is both environmentally safe and environmentally friendly.
Further, a need exists for the development of a high temperature retarder which delays setting of a cement slurry at bottom hole temperatures in excess of 350° F.